The present invention relates to a semiconductor memory device, and more particularly to a dynamic random access memory (hereinbelow, abbreviated to "D-RAM").
In order to make a semiconductor memory device of the MIS (Metal-Insulator-Semiconductor) type high in the operating speed and high in the integration density or packaging density, it has been considered to employ a high-fusing metal, that is, a refractory metal, such as molybdenum (Mo), tantalum (Ta) and tungsten (W), as the material of the gate electrodes of the memory device. Such metal materials have the advantage that they are lower in resistivity than polycrystalline silicon etc., and the advantage that in forming source and drain regions, they can be self-aligned likewise to polycrystalline silicon. Therefore, they are very effective for realizing the high operating speed and high packaging density of the semiconductor memory device.
Especially in the D-RAM, it is eagerly requested to achieve high operating speed and high packaging density. A D-RAM employing the aforementioned metal material is described in, for example, a literature "IEEE Transactions on Electron Devices, Vol. ED-27, No. 8, August 1980", p. 1602-1606. According to the literature, a polycrystalline silicon layer which is a first layer of wiring is utilized as the gate electrode of each of the insulated gate type field effect transistors (hereinbelow, termed "MISFETs") of a peripheral circuit for operating a memory cell and as one electrode of the charge storing capacitor of the memory cell, and a molybdenum layer which is a second layer of wiring is used as the gate electrode of a MISFET in the memory cell, that is, as a word line.
That is to say, the molybdenum layer is used only within a memory array which is constructed of a plurality of memory cells.
In applying such refractory metal, the inventors took notice of latent problems as follows:
The refractory metal film is inferior in the close adhesion with an SiO.sub.2 film etc. Impurities such as sodium ions which mix in a manufacturing process are liable to penetrate through the metal film, and they reach the interface between Si and SiO.sub.2 and varies a threshold voltage (V.sub.th), so that a MISFET having stable characteristics cannot be obtained. Particularly in a MISFET which constitutes a peripheral circuit such as sense amplifier requiring to reliably amplify a minute signal, unlike a MISFET within a memory cell, it was improper to use only the refractory metal as the gate electrode of the MISFET.
Further, notwithstanding that a wiring layer made of the refractory metal is formed on a field insulating film having been formed by applying the LOCOS (Local Oxidation of Silicon) technique which affords a gentle step, the same refractory metal is employed for the second wiring layer, and hence, breaking is prone to occur in the place of the second wiring layer intersecting with the first wiring layer.
The present invention has been made on the basis of the notice of the latent problems described above.